Fluctuation readings from a vacuum gauge at idle generally indicates a leaking intake valve or leaking inlet manifold gasket. It can sometimes be slightly different depending on where the vacuum gauge in connected. However a cam lobe or lifter problem is less likely but can't be ruled out as is a riding rocker. Large cam lobe overlap and unsteady idle will also give fluctuations, and correct checking of the readings on a vacuum gauge can indicate the difference in the problems I have mentioned above. But that can take a reasonable amount of experience with reading vacuum gauges which seams to be a bit of a lost art these days. Tuning engines with a vacuum gauge before EFI was introduced was the norm. I can't see your car from here, but just reading between the lines I wouldn't rule out the brake booster. It sounds like you may have two problems, the master cylinder which has now been rectified and a vacuum problem (either engine or booster)

Quick test at home,

Run the engine and get full vacuum, turn the engine off, place your foot on the brake pedal with moderate steady pressure and hold.
The brake pedal feel shouldn't feel any change over a minute. If the pedal starts to feel like its trying to force your foot up (push back) then you have a problem with the brake booster or booster inlet check valve.

If after one minute the feel doesn't change take your foot off the pedal and slowly pump the pedal up and down a further three or four times. Each time you pump the pedal it should feel a little bit harder to push down. (using vacuum) After that the pedal will feel stiff and harder to depress. This will indicate your booster is working ok. If you can't get at least three pumps before all the booster vacuum is lost then you have a booster problem. Note, you need to have full vacuum in the booster at the time of turning the engine off. If you can get a hold of a vacuum gauge and place it between the booster inlet check valve and the booster it self, then the above test can be more accurately viewed. I have a brass T piece in my tool box that I have used over the past forty years to plug between the booster a check valve, its come in bloody handy over the years for just this test.